The present invention pertains to the determination of the purity of a gas in a gas mixture and, more particularly, to a method and apparatus for determining the purity of a first gas in a binary mixture with another known gas which mixture is being circulated in a closed system by a fan.
Binary mixtures of two gases are often utilized in various kinds of closed systems, for example, to process certain materials therein or to provide a medium for heat transfer. As an example of the latter, a hydrogen gas with an air impurity fraction is often used to cool large AC generators by circulating the gas mixture within the generator housing. Hydrogen, in particular, has a very high thermal conductivity which makes it possible to run large generators at higher loads because the circulating hydrogen provides better removal of heat from the generator windings. The thermal conductivity of hydrogen is six to seven times greater than air and, therefore, as high a percentage of hydrogen as practical in the cooling gas mixture is most desirable.
Because of the highly combustible nature of hydrogen, the volume percent or purity of hydrogen in the mixture must also be maintained relatively high. It is desirable in this heat transfer application to maintain the purity of hydrogen in the mixture above 90% and, more typically, above a minimum of 95%. Obviously, because of the practical impossibility of obtaining pure hydrogen gas and the presence of normal system leakage, a pure hydrogen atmosphere cannot be maintained. Nevertheless, hydrogen purities as high as 98% to 99% are fairly readily attainable.
It is also necessary, in order to take best advantage of the high thermal conductivity of hydrogen to maintain an adequate pressure within the system. In a large AC generator, for example, operation at full load requires the maintenance of a system pressure up to about 75 psi. Typical hydrogen leakage through the generator shaft seals or by absorption in the lubricating oil requires the regular addition of makeup hydrogen, both to maintain system pressure and to maintain hydrogen purity at a safe high level.
Because of the potential hazards which are attendant a reduction in hydrogen purity below safe, noncombustible levels, appropriate purity monitoring and alarm systems must be utilized. The primary means of monitoring hydrogen purity in the cooling gas mixture inside a large AC generator is by the use of a thermal gas analyzer. Because the relative conductivity of hydrogen is so much greater than the air or other gas impurity with which it is typically mixed, thermal conductivity is an excellent indicator of hydrogen purity. Thus, a thermal gas analyzer is typically used to continuously monitor hydrogen purity and to provide an appropriate alarm if the purity drops below established safe levels. However, typical thermal gas analyzers have a notoriously slow response time and it is not unusual for the output of a thermal gas analyzer to lag an actual system change in hydrogen purity by as much as one hour. Obviously, the alarm signal will also lag correspondingly the actual alarm condition. In addition, a typical thermal gas analyzer requires the use of a dryer in the sample feedline to remove moisture from the gas sample prior to analysis. With water vapor removed from the air/hydrogen sample being analyzed, the sample is not actually representative of the gas mixture used in the generator cooling system
An independent back-up means of determining hydrogen purity is typically provided by the use of a direct reading manometer calibrated to give a rough indication of hydrogen purity in the generator. The manometer is calibrated empirically based on measured gas purities at various measured machine pressures. A typical large generator includes two shaft-mounted fans inside the housing to circulate the cooling hydrogen/air mixture. Differential fan pressure is monitored directly with the appropriately calibrated manometer which also provides a much more rapid response to changes in hydrogen purity than does a thermal gas analyzer. However, a manometer provides only a rough indication of hydrogen purity and is typically difficult to read accurately. Moreover, a manometer cannot be readily connected to be read or monitored at a remote location nor can it be easily adapted to be connected to an alarm system.
There is, therefore, a need for a method and apparatus for determining the purity of one gas in a known binary gas mixture which is accurate and operates on an essentially real time basis. In particular, it would be desirable to have a method and means for accurately and rapidly monitoring hydrogen purity in the coolant gas mixture circulating within a large generator or the like. Further, it would be desirable to have a method in which the water vapor fraction in a binary gas mixture could be properly included in determining the actual purity or volume fraction of either of the two gases.